1. Field of the Invention
The present invention relates to a lens drive apparatus which is suitable for, for example, an image pickup apparatus such as a video camera or a digital camera which can drive a movable lens frame movable in an optical axis direction in a lens barrel with high accuracy.
2. Description of the Related Art
A lens barrel of an image pickup apparatus such as a video camera having an automatic focusing function or a power zoom function generally includes a lens drive apparatus for moving a focusing or zooming movable lens frame in an optical axis direction.
A conventionally known drive unit in this case includes, for example, a lens drive apparatus using an electromagnetic drive actuator including a coil and a magnet, that is, an electromagnetic drive apparatus (Japanese Patent Application Laid-Open No. 2002-169073).
FIG. 6 is an exploded perspective view of an electromagnetic drive apparatus for driving a focus lens in an optical axis direction disclosed in Japanese Patent Application Laid-Open No. 2002-169073.
The electromagnetic drive apparatus in FIG. 6 includes a magnet 102 fixedly supported by an unshown fixed outer barrel member, a movable lens frame 104 driven slidably in the optical axis direction, and a coil (movable coil) 103 fixedly supported by the movable lens frame 104.
Areas 102A and 102B that constitute the magnet 102 have magnetic poles magnetized oppositely in parallel. In a configuration of the electromagnetic drive apparatus in FIG. 6, a first yoke 101 and a second yoke 105 made of magnetic material draw opposite magnetic fluxes of the areas 102A and 102B generated by the magnet 102, and a vertical magnetic field can be provided to the coil 103.
Electromagnetic driving by the coil 103 and the magnet 102 will be described below.
The magnet 102 has a magnetic flux density of the areas 102A and 102B magnetized oppositely in parallel, while the coil 103 generates a magnetic field according to a current flow direction.
The magnetic flux of the magnet 102 passing through the coil 103 is desirably substantially perpendicular to an optical axis direction.
The magnetic field generated by applying a current to the coil 103 and the magnetic field generated by the magnet 102 produce thrust for driving in the optical axis direction (Fleming's left-hand rule).
The movable lens frame 104 integrated with the coil 103 is moved in the optical axis direction by the thrust.
In FIG. 6, a sleeve 104A is provided in the member to be driven 104. A bifurcated hook arm 104C is provided on a lower side of the member to be driven 104.
The electromagnetic drive apparatus having the configuration in FIG. 6 has a superior driving property. In FIG. 6, the second yoke 105 is placed to face the magnet 102 so as to hold the coil 103 therebetween as its function.
Thus, as shown in FIG. 6, the coil 103 is significantly spaced from the movable lens frame 104 perpendicularly to the optical axis, and the second yoke 105 is passed through the space. In the configuration in FIG. 6, the second yoke 105 is used and thus the entire electromagnetic drive apparatus tends to be increased in size.
FIG. 7 illustrates a driving property of the electromagnetic drive apparatus with the second yoke 105 being omitted in the electromagnetic drive apparatus in FIG. 6.
The electromagnetic drive apparatus in FIG. 7 does not include the second yoke 105 for drawing the magnetic flux generated by the magnet 102. Thus, the magnetic flux passing through the coil 103 does not form a vertical magnetic field as shown in FIG. 7, and thus a propulsion force is generated in a direction other than the optical axis direction indicated by an arrow MO according to a direction of a current flow through the coil 103.
The propulsion force generated in the direction other than the optical axis direction may prevent driving the movable lens frame 104 in the optical axis direction. The propulsion force may also lift the movable lens frame 104 when a driving direction is reversed. This may cause blur of picked-up images or generates abnormal noise.
The present invention has an object to provide a lens drive apparatus and an image pickup apparatus including the lens drive apparatus that can prevent a propulsion force generated in a direction other than an optical axis direction without a second yoke, reduce a size of the entire apparatus, and drive a lens holding frame in the optical axis direction with high accuracy.